A highly efficient luminescent material has been developed for reducing the power consumption of an organic EL (electroluminescence) element. A phosphorescent luminescent material using the emission from the triplet excited state can achieve a high luminous efficiency compared to a fluorescent luminescent material using only the fluorescent emission from the singlet excited state. Therefore, a phosphorescent luminescent material has been developed.
Currently, a phosphorescent material capable of achieving an internal quantum efficiency of approximately 100% at a maximum is used for green pixels and red pixels of an organic EL element. However, a fluorescent material having an internal quantum efficiency of approximately 25% at a maximum is used for blue pixels. The reason is that blue light emission requires a higher energy than that of red light or green light emission; and when it is attempted to obtain high-energy emission from phosphorescent emission at the triplet excited level, portions in a molecular structure which are unstable under high energy are likely to deteriorate.
As a blue phosphorescent material, in order to achieve a high-energy triplet excited state, an iridium (Ir) complex in which an electron-attracting group such as fluorine is introduced into a ligand as a substituent is known (for example, refer to NPLs 1 to 5). However, in a blue phosphorescent material into which an electron-attracting group is introduced, the luminous efficiency is relatively high, whereas the light resistance is low and the lifetime is short.
In addition, it is reported that short-wavelength emission is possible in a complex in which a carbene ligand is used without introducing an electron-attracting group thereinto (refer to NPL 6 and PTL 1).